This invention relates to plasma display device in general and more particularly to such a device with improved operating characteristics.
A plasma display device comprising a gastight housing, the interior of which contains a gas discharge space which is filled with an ionizable gas at a predetermined pressure and in which an electron and/or photon generating gas discharge is developed between at least one cathode of aluminum with, optionally, a small amount of other elements and at least one further electrode, as well as means for addressing the picture elements of a flat picture screen is disclosed in U.S. Pat. No. 3,956,667.
Display devices with a flat picture screen which can, for instance, replace the heretofore known color television tubes, frequently contain a planar electron source. The individual picture elements on the picture screen are then excited by electrons controlled by means of a matrix like addressing system. The electrons required therefore are in some cases produced directly in a gas discharge between a planar electrode and a further electrode. However, a photocathode following the gas discharge path can also serve as the electron source. In that case, electrons are released by photons which have been generated in the gas discharge. With such planar gas discharge cathodes, electrons with uniform density can be produced directly or indirectly. These electrons are relatively slow and their intensity can therefore easily be controlled.
Such a cathode of a display device with a flat picture screen is known, for instance, from U.S. Pat. No. 3,956,667 mentioned above. This device, in which a gas discharge is produced, contains auxiliary anodes for controlling the lines as well as the control electrodes for addressing the individual picture elements of a line which is turned on. In the interior of its gastight housing, which is under the predetermined pressure of a suitable filling gas such as argon or neon, a gas discharge path between a large area cathode and the auxiliary anodes, as well as an electron accelerating path between the control electrodes and an anode, are produced. A hole matrix formed by a plate of insulating material divides the common interior of the housing into a gas discharge space with a relatively great length for operation with low voltage for the gas discharge current, and a second space of short length and high field strength for accelerating the electrons. The auxiliary anodes associated with the lines of the matrix are arranged on the one flat side of the plate of insulating material serving as the hole matrix, and the control electrodes for addressing the picture elements are arranged on the opposite flat side. The electrons, which are generated in a glow discharge controlled line by line and which are moved toward the corresponding auxiliary electrode, are controlled point-by-point in the following electron accelerating section of high field strength by the correspondingly divided control electrode. There, they are accelerated toward the anode and are utilized on its picture screen for the excitation of defined picture elements. The anode is designed as a coherent, large area luminescent screen electrode. If a line of the auxiliary electrodes is addressed, the discharge burns uniformly along the entire line anode, while the so-called negative glow light of the gas discharge covers a region, the area of which is determined by the known dependence of the current density on the chosen cathode gas system and the gas pressure.
Instead of a single areal cathode, several subcathodes can also be provided for the known display device, to which a respective group of auxiliary anodes is assigned (U.S. Pat. No. 4,130,778).
In another known plasma display device, a glow discharge produced between two discharge electrodes is brought about for generating the electrons. Aluminum is provided as the material for the cathode (U.S. Pat. No. 3,622,829).
In a plasma display device, a high vacuum tight separation of the electron acceleration space from the gas discharge space may also be provided. To separate these two spaces, a light transparent partition is provided, having its side facing the picture screen provided with a photocathode as the electron source. The so-called negative glow light in the gas discharge space is then used for generate the photons which excite the photocathode to emit electrons (DE-OS No. 26 56 621).
A gas discharge suitable for these display devices must meet a number of requirements. One main requirement is to provide a suitable system of filling gas and electrodes in which, on the one hand, a gas discharge with sufficient electron yield is possible but, on the other hand, a system in which a breakdown in the electron post acceleration space, which is, for instance, at the same pressure as the gas discharge space, is prevented. In addition, an operating voltage which is as low as possible and is approximately constant should be required, since this simplifies the electrical controllability of the picture screen. In addition, it is necessary that little atomization of the cathode take place, i.e., that the sputter yield be low. Sputter yield is understood here to be the number of cathode atoms knocked out by positive ions by cathode sputtering, divided by the number of ions impinging on the cathode. For, a large amount of material removal at the cathode by sputtering can have an adverse effect on the electrical parameters of the gas discharge such as the operating voltage and the current density, for instance, through destruction of a surface layer advantageous for the gas discharge. In the case of electrically conductive deposits, furthermore, there is a danger of short circuits in the display device. On the other hand, electrically non-conducting sputter deposits on an anode or on auxiliary anodes can lead to a cutoff of the same.
It is therefore an object of the present invention to provide a display device of the type mentioned at the outset, in which these requirements are at least largely met. In particular, a system consisting of a gas discharge plasma and cathode is to be provided that leads to only a low sputter yield and in which the unavoidable sputter deposits are electrically non-conducting. In addition, the electrical parameters of the gas discharge should not change substantially during a required life of about 5,000 operating hours or more.